1. Field of the Invention
The present invention relates generally to a prism optical system, and more specifically to a prism optical system having power, which comprises a decentered reflecting surface.
2. Background of Related Art
Among reflecting decentered optical systems of compact size known in the art, there is a one-dimensional light receiving lens system comprising a cylindrical reflecting surface as disclosed in JP-A-59-84201. However, this system can never take two-dimensional images. JP-A-62-144127 describes a system using an identical surface for two reflections, thereby reducing spherical aberration produced in the system described in JP-A-59-84201.
JP-A-62-205547 shows the use of an aspheric form of reflecting surface, but fails to refer exactly to the configuration of the reflecting surface. U.S. Pat. Nos. 3,810,221 and 3,836,931 disclose examples of a lens system comprising a rotationally symmetric aspheric mirror and a surface having only one symmetric plane, which is used for a finder optical system for reflex cameras. Note that the surface having only one symmetric plane is used for the purpose of correcting the tilt of the image to be observed.
JP-A-1-257834 (corresponding to U.S. Pat. No. 5,274,406) discloses an example of using a surface having only one symmetric plane for a reflector to make correction for image distortion in a rear projection type television, wherein a projection lens system is used for projection onto a screen, and the surface having only one symmetric plane is used for correcting image distortion.
JP-A-7-333551 discloses an example of a back-surface type of decentered optical system which comprises an anamorphic surface and a toric surface, and is designed as a viewing optical system. However, this system is still less than satisfactory in terms of correction of aberrations inclusive of image distortion, and so cannot be applied to a phototaking optical system.
However, none of these prior systems is a system having only one symmetric plane and using a back-surface mirror for a turn-back optical path.
Also, all these prior systems are directed to a sort of rotationally symmetric optical system wherein a refracting lens forming part of the optical system is constructed of a plane rotationally symmetric with respect to an optical axis.
One problem with such prior systems is that unless a formed real image is well corrected for aberrations and distortion, an image pattern or the like will be distorted, ending up with a failure in recording an exact form thereof. Another problem is that since the optical path is in a linear form, the optical system becomes long in the optical axis direction, resulting in an increase in phototaking device size.